Typically, output driver circuits have several parallel output stages that are switched in a staggered fashion to provide programmable rise and fall times. FIG. 1 illustrates a graph of driver output voltage as a function of time for an ideal case of a low to high transition. The curve 10 has a completely smooth transition from −1 to +1 volts during the time period from approximately t=2 milliseconds (ms) to approximately t=8 ms. The smooth transition shown in FIG. 1 does not occur in the non-ideal case.
FIG. 2 illustrates a graph of driver output voltage as a function of time for a non-ideal case of a transition from low to high for a driver circuit that employs 3-stage staggered switching. In the non-ideal case represented by the graph shown in FIG. 2, three steps 21, 22 and 23 occur during the transition from −1 to +1 volts during the time period from approximately 4 ms to approximately 8 ms.
It is desirable to smooth the steps in the transition regions. One way to ensure that the steps in the transition region are smooth is to use a distributed resistor-capacitor (RC) circuit in the driver circuit to provide the desired rise and fall transition times in the driver output signal. However, distributed RC circuits have limited capabilities because they generally are not programmable.
Another way to smooth the steps in the transition regions is to increase the number of stages that are staggered in the driver circuit. Currently, attempts are being made to increase the range of rise and fall times that can be provided by driver circuits. In order to smooth the steps in the transition regions of driver circuits that have wider ranges of rise and fall times, an even greater number of staggered stages are needed. Of course, increasing the number of staggered stages increases the complexity of the driver circuit as well as the amount of area that is needed to implement the driver circuit in an IC and the amount of power that the driver circuit consumes.
A need exists for a driver circuit that is capable of providing a wide range of programmable rise and fall times with relatively smooth steps in the transition regions. A need also exists for a driver circuit that has such capabilities and that is capable of being implemented in an IC in a relatively small area with relatively low complexity.